Increasingly stringent statutory regulations in relation to permissible pollutant emissions from motor vehicles in which internal combustion engines are disposed make it necessary to keep the pollutant emissions produced during the operation of the internal combustion engine to an absolute minimum. This can be achieved firstly by reducing the pollutant emissions that are produced during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. Secondly, use is also made in internal combustion engines of exhaust gas aftertreatment systems which convert the pollutant emissions that are generated during the combustion process of the air/fuel mixture in the respective cylinders into harmless substances. For that purpose exhaust gas catalytic converters are used which convert carbon monoxide, hydrocarbons and nitrogen oxides into harmless substances. Both the targeted influencing of the generation of the pollutant emissions during combustion and the converting of the pollutant components with a high degree of efficiency by means of an exhaust gas catalytic converter are predicated on a very precisely adjusted air/fuel ratio in the respective cylinder.
In particular in connection with an increasing trend to dispose the exhaust gas catalytic converters very close to the engine, a precise, cylinder-specific adjustment of the air/fuel ratio is also increasingly important, since the individual exhaust gas packages intermix only relatively poorly due to the short mixing section. For cost reasons, however, only one exhaust gas probe is generally disposed upstream of the respective exhaust gas catalytic converter at the respective exhaust gas branch of the exhaust gas tract.
For the purpose of a cylinder-selectively precise setting of the respective air/fuel ratio in the respective combustion chambers of the respective cylinders it is known from DE 10 2004 004 291 B3 to use a cylinder-selective lambda control by means of which the individual deviations of the respective cylinder-selective air/fuel ratios are to be minimized to an average air/fuel ratio. A measured signal of an exhaust gas probe disposed in an exhaust gas tract, which signal is characteristic of the air/fuel ratio in the respective cylinder, is recorded at a predefined crankshaft angle in relation to a reference position of the piston of the respective cylinder and assigned to the respective cylinder. An actuating variable for influencing the air/fuel ratio in the respective cylinder is generated by means of the cylinder-individual lambda controller as a function of the measured signal recorded for the respective cylinder. The predefined crankshaft angle is adjusted as a function of an instability criterion of the controller.
From DE 103 04 245 B3 it is known to employ an optimization method for improving the control behavior of a cylinder-individual lambda control with regard to an adaption; by varying the sampling points for recording the measured signal of the exhaust gas probe both in the positive and in the negative direction, the method optimizes a characteristic which is indicative of the deviation of the individual measured lambda values.